1. Field of the Invention
This invention relates to printed circuit which is made by electronic circuits such as digital circuits on printed circuit board and more particularly to a both surface printed circuit provided with a ground plane so as to inhibit a noise generating phenomenon on the ground plane by a radiation from a signal pattern and to make it possible to package a circuit with little noise.
2. Description of the Related Art
The high speed circuit processing has recently advanced in digital circuits used in electronic devices. When the circuit is processed at a higher speed, clock signals and other signals will be made higher in the frequency and an unnecessary radiation will be generated by the influence of their harmonic waves.
As a countermeasure against it, there is a method of inhibiting the unnecessary radiation by adopting a so-called ground plane forming a ground pattern as a surface by using a both surface or multilayer printed circuit for a printed circuit forming a digital circuit.
That is to say, as shown in FIG. 6, circuit devices 2 and 3 each having a plurality of terminals are fitted to one surface (which shall be called a part surface hereinafter) of an insulating board 1, an output terminal 2b of the circuit device 2 and an input terminal 3b of the circuit device 3 are connected with each other through a signal pattern 4 so that an unnecessary radiation generated from this signal pattern 4 may be inhibited by a ground plane 5. This drawing is of an example of a both surface printed circuit board in which the ground plane 5 is formed on the other surface of the insulating board 1 and such terminals which must not be grounded as the signal output terminal 2b and input terminal 3b are prevented from being connected with the ground plane 5 by holes (cutout holes) 7 made by cutting out parts of the ground plane 5. Also, on the part surface, a ground pattern 6 made to be of the same potential as of a ground plane 5 is formed of jumper wires or through holes. The ground terminals 2a and 3a of the circuit devices 2 and 3 are passed through the cutout holes 7 formed in the ground plane 5 so as not to be connected directly with the ground plane 5.
However, in case the ground plane 5 is provided as mentioned above, a high frequency current flowing through the signal pattern 4 will flow as a normal mode noise through the ground plane 5 through a stray capacitor C.sub.1 produced between the signal pattern 4 and ground plane 5 shown in FIG. 7 or will flow as a common mode noise through the ground pattern 6. In such case, as other circuit devices are also fitted to the printed circuit board a high frequency current from these devices will be added and thus, if a great high frequency current flows, the potentials of the ground pattern 6 and ground plane 5 will produce a high frequency potential difference, thereby the potential of the ground plane 5 will become unstable and, depending on the high frequency current, an unnecessary radiation will be thought to be caused from the ground part.
As explained above, in the conventional printed circuit in which the ground plane 5 is formed to inhibit the unnecessary radiation, there has been a problem that the high frequency current flowing through the signal pattern 4 will flow through the ground plane 5 and the ground pattern 6 on the same plane and the potential difference between both grounds will become unstable to cause an unnecessary radiation.